One-pot freezing-thawing preparation of cellulose nanofibrils reinforced polyvinyl alcohol based ionic hydrogel strain sensor for human motion monitoring

Anisotropic conductive eutectogels for strain sensing and triboelectric nanogeneration in extreme environments

Conductive hydrogels have attracted widespread attention for their promising application prospects in portable and flexible electronic devices. However, hydrogels commonly suffer from problems such as solvent volatilization and freezing at low temperatures. Inspired by tissues such as human muscles, tendons, and ligaments, this study proposes a facile method to produce anisotropic conductive strong and tough eutectogels through directional freezing integrated with solvent substitution (DFSS) strategy. Eutectogels with anisotropic characteristics exhibit a highly anisotropic structure, conferring distinctive anisotropic mechanical properties and electrical conductivity. The prepared anisotropic PVA-M-DES eutectogels exhibit excellent mechanical properties (high strength of 6.31 MPa, high toughness of 20.75 MJ m-3, elastic modulus of 2.36 MPa, and fracture strain of 596%), high conductivity (0.17 S m-1), excellent anti-freezing and anti-drying properties. Environment-tolerant anisotropic PVA-M-DES eutectogels can be assembled into strain sensor and triboelectric nanogenerator to achieve real-time monitoring of various human motions and have potential applications in wearable electronics, personal healthcare, energy harvesting, and human-machine interfaces.

Self-healing cellulose-based hydrogels: From molecular design to multifarious applications

Self-healing cellulose-based hydrogels (SHCHs) exhibit wide-ranging potential applications in the fields of biomedicine, environmental management, energy storage, and smart materials due to their unique physicochemical properties and biocompatibility. This review delves into the molecular design principles, performance characteristics, and diverse applications of SHCHs. Firstly, the molecular structure and physicochemical properties of cellulose are analyzed, along with strategies for achieving self-healing properties through molecular design, with particular emphasis on the importance of self-healing mechanisms. Subsequently, methods for optimizing the performance of SHCHs through chemical modification, composite reinforcement, stimulus responsiveness, and functional integration technologies are discussed in detail. Furthermore, applications of SHCHs in drug delivery, tissue engineering, wound healing, smart sensing, supercapacitors, electronic circuits, anti-counterfeiting systems, oil/water separation, and food packaging are explored. Finally, future research directions for SHCHs are outlined, including the innovative development of new SHCHs, in-depth elucidation of cooperative strengthening mechanisms, a further expansion of application scope, and the establishment of intelligent systems. This review provides researchers with a comprehensive overview of SHCHs and serves as a reference and guide for future research and development.

Highly stretchable, adhesive and conductive hydrogel for flexible and stable bioelectrocatalytic sensing layer of enzyme-based amperometric glucose biosensor

Highly stretchable, adhesive and conductive triblock hydrogel was synthesized and utilized as a flexible and stable bioelectrocatalytic sensing layer of enzyme-based amperometric glucose biosensor. The hydrogel was prepared through one-pot polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid, methacrylamide, and hydroxyethyl methacrylate. The physical and chemical properties of the hydrogel were characterized with X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and electrochemical techniques. Glucose oxidase (GOx) and chitosan (CTS) embedded hydrogel was drop-coated on glassy carbon electrode (GCE) and screen printed graphite electrode (SPGE). The resulting GOx/CTS/hydrogel-GCE and GOx/CTS/hydrogel-SPGE exhibited excellent mediated bioelectrocatalytic oxidation current for glucose. The calibration curve of glucose by the GOx/CTS/hydrogel-GCE showed the linear range from 0.25 to 15 mM with the sensitivity of 27.0 µA mM-1 cm-2. This GOx/CTS/hydrogel-based sensing layer coated on the SPGE was stable against bending, and the response to glucose was almost same irrespective of the bending angles (0, 30, 60, and 90 degree). In addition, the response to glucose was not interfered by various organic and inorganic interfering species, allowed to detect glucose in goat serum. Furthermore, the GOx/CTS/hydrogel-GCE kept its original activity of 99.64 % during 30 days' storage under dry state in refrigerator.

Fishing net-inspired PVA-chitosan-CNT hydrogels with high stretchability, sensitivity, and environmentally stability for textile strain sensors

Soft electronic products are being extensively investigated in diverse applications including sensors and devices, due to their superior softness, responsiveness, and biocompatibility. One-dimensional (1-D) fiber electronic devices are recognized for their lightweight, wearable, and stretchable qualities, thus emerging as critical constituents for seamless integration with the human body and attire, exhibiting great potential in wearable applications. However, wearable conductive hydrogel fibers usually face challenges in combining stretchability and excellent stability, notably in high-temperature environment. Herein, a novel stretchable conductive hydrogel fiber, namely PVA-CS-CNT (Polyvinyl Alcohol-Chitosan-Carbon Nanotube) hydrogel fiber, was successfully prepared through a straightforward low-temperature process. This hydrogel fiber not only maintains stable signal transmission at high temperatures but also exhibits significant mechanical and sensing capabilities, ensuring signal stability during repetitive cyclic stretching. Inspired by fishing net, textile sensors were fabricated by weaving PVA-CS-CNT hydrogel fibers, which offered breathability, high stability (withstanding over 500 stretch cycles), high sensitivity (detecting strains as low as 1 %), and exceptional mechanical strength (exceeding 17 MPa). The wearable sensor could not only accurately monitor human movements like stretching and bending, but also adeptly captured delicate signals such as pulses and sounds. These characteristics demonstrated the potential applications of the hydrogel fibers encompassing human motion tracking, intelligent textiles, and soft robotics.

A tough, stretchable, adhesive and electroconductive polyacrylamide hydrogel sensor incorporated with sulfonated nanocellulose and carbon nanotubes

Recently, hydrogel sensors have been widely applied in wearable and portable electronics, but the low mechanical property, intolerance of fatigue, and low sensitivity and adhesion limit their further applications. In this study, sulfonated nanocellulose (SCNF) with dual functionality was blended into polyacrylamide (PAM) hydrogel matrix to reinforce the mechanical strength and facilitate the homogeneous dispersion of carbon nanotubes (CNTs). The SCNF-CNT/PAM hydrogel was designed through free radical polymerization to achieve commendable mechanical, electrical, and multifunctional properties. The environmental-friendly SCNF serves as bio-templates to facilitate the assembling of CNT into integrated SCNF-CNT structures with good dispersity, thus enabling the establishment of an integrated conducting and reinforcing network. The fabricated SCNF-CNT/PAM hydrogel exhibited outstanding compressive strength (∼0.45 MPa at 50 % strain), tensile strength (∼169.12 kPa), and antifatigue capacity under cyclic stretching and pressing. Furthermore, the multifunctional sensors assembled using this hydrogel demonstrated high strain sensitivity (gauge factor ~ 3.7 at 100-400 % strain) and effectively detected human motions. This design principle provides promising prospects for constructing next-generation multifunctional flexible sensors, and the integration of these distinctive properties enables the prepared composite hydrogels to find potential applications in various areas, such as implantable soft electronic devices, electronic skin, and human movement monitoring.

Attapulgite-Reinforced Cellulose Hydrogels with High Conductivity and Antifreezing Property for Flexible Sensors

Ionic conductive cellulose hydrogels are some of the most promising candidates for flexible sensors. However, it is difficult to simultaneously prepare cellulose hydrogels with high mechanical strength, good ionic conductivity, and antifreeze performance. In this work, a natural clay (attapulgite)-reinforced cellulose hydrogel was fabricated. Through a one-pot method, cellulose and attapulgite were dispersed in a concentrated ZnCl2 solution. The obtained hydrogel exhibited a dual network of hydrogen bonds and Zn2+-induced ionic interactions. Attapulgite serves as an inorganic filler that can regulate the hydrogen-bonding density among cellulose molecules and provides abundant channels for fast ion transport. By optimizing the attapulgite loading, a mechanically strong (compressive strength up to 1.10 MPa), tough (fracture energy up to 0.36 MJ m-3), highly ionic conductive (4.15 S m-1), and freezing-tolerant hydrogel was prepared. These hydrogels can be used for sensitive and stable human motion sensing, demonstrating their great potential for healthcare applications.

High-strength, anti-fatigue, cellulose nanofiber reinforced polyvinyl alcohol based ionic conductive hydrogels for flexible strain/pressure sensors and triboelectric nanogenerators

Ionic conductive hydrogels (ICHs) have attracted great attention because of their excellent biocompatibility and structural similarity with biological tissues. However, it is still a huge challenge to prepare a high strength, conductivity and durability hydrogel-based flexible sensor with dual network structure through a simple and environmentally friendly method. In this work, a simple one-pot cycle freezing thawing method was proposed to prepare ICHs by dissolving polyvinyl alcohol (PVA) and ferric chloride (FeCl3) in cellulose nanofiber (CNF) aqueous dispersion. A dual cross-linked network was established in hydrogel through the hydrogen bonds and coordination bonds among PVA, CNF, and FeCl3. This structure endows the as-prepared hydrogel with high sensitivity (pressure sensitivity coefficient (S) = 5.326 in the pressure range of 0-5 kPa), wide response range (4511 kPa), excellent durability (over 3000 cycles), short response time (83 ms) and recovery time (117 ms), which can accurately detect various human activities in real time. Furthermore, the triboelectric nano-generator (TENG) made from PVA@CNF-FeCl3 hydrogel can not only supply power for commercial capacitors and LED lamps, but also be used as a self-powered sensor to detect human motion. This work provides a new approach for the development of the next generation of flexible wearable electronic devices.

High-performance and frost-resistance MXene co-ionic liquid conductive hydrogel printed by electrohydrodynamic for flexible strain sensor

Conductive hydrogels with high performance and frost resistance are essential for flexible electronics, electronic skin, and soft robots. Nonetheless, the preparation of hydrogel-based flexible strain sensors with rapid response, wide strain detection range, and high sensitivity remains a considerable challenge. Furthermore, the inevitable freezing and evaporation of water in sub-zero temperatures and dry environments lead to the loss of flexibility and conductivity in hydrogels, which seriously limits their practical application. In this work, ionic liquids (ILs) and MXene are introduced into gelatin/polyacrylamide (PAM) precursor solution, and a PAM/gelatin/ILs/MXene/glycerol (PGIMG) hydrogel-based flexible strain sensor with MXene co-ILs ion-electron composite conductive network is prepared by combining the electrohydrodynamic (EHD) printing method and in-situ photopolymerization. The introduction of ILs provides an ionic conductive channel for the hydrogel. The introduction of MXene nanosheets forms an interpenetrating network with gelatin and PAM, which not only provides a conductive channel, but also improves the mechanical and sensing properties of the hydrogel-based flexible strain sensor. The prepared PGIMG hydrogel with the MXene co-ILs ion-electron composite conductive network demonstrates a tensile strength of 0.21 MPa at 602.82 % strain, the conductivity of 1.636 × 10-3 S/cm, high sensitivity (Gauge Factor, GF = 4.17), a wide strain detection range (1-600 %), and the response/recovery times (73 ms and 74 ms). In addition, glycerol endows the hydrogel with excellent freezing (-60 °C) and water retention properties. The application of the hydrogel-based flexible strain sensor in the field of human motion detection and information transmission shows the great potential of wearable devices, electronic skin, and information encryption transmission.

Data-Driven Strain Sensor Design Based on a Knowledge Graph Framework

Wearable flexible strain sensors require different performance depending on the application scenario. However, developing strain sensors based solely on experiments is time-consuming and often produces suboptimal results. This study utilized sensor knowledge to reduce knowledge redundancy and explore designs. A framework combining knowledge graphs and graph representational learning methods was proposed to identify targeted performance, decipher hidden information, and discover new designs. Unlike process-parameter-based machine learning methods, it used the relationship as semantic features to improve prediction precision (up to 0.81). Based on the proposed framework, a strain sensor was designed and tested, demonstrating a wide strain range (300%) and closely matching predicted performance. This predicted sensor performance outperforms similar materials. Overall, the present work is favorable to design constraints and paves the way for the long-awaited implementation of text-mining-based knowledge management for sensor systems, which will facilitate the intelligent sensor design process.

Nanocellulose-based hydrogels as versatile materials with interesting functional properties for tissue engineering applications

Tissue engineering has emerged as a remarkable field aiming to restore or replace damaged tissues through the use of biomimetic constructs. Among the diverse materials investigated for this purpose, nanocellulose-based hydrogels have garnered attention due to their intriguing biocompatibility, tunable mechanical properties, and sustainability. Over the past few years, numerous research works have been published focusing on the successful use of nanocellulose-based hydrogels as artificial extracellular matrices for regenerating various types of tissues. The review emphasizes the importance of tissue engineering, highlighting hydrogels as biomimetic scaffolds, and specifically focuses on the role of nanocellulose in composites that mimic the structures, properties, and functions of the native extracellular matrix for regenerating damaged tissues. It also summarizes the types of nanocellulose, as well as their structural, mechanical, and biological properties, and their contributions to enhancing the properties and characteristics of functional hydrogels for tissue engineering of skin, bone, cartilage, heart, nerves and blood vessels. Additionally, recent advancements in the application of nanocellulose-based hydrogels for tissue engineering have been evaluated and documented. The review also addresses the challenges encountered in their fabrication while exploring the potential future prospects of these hydrogel matrices for biomedical applications.

Polydopamine-Anchored Cellulose Nanofiber Flexible Aerogel with High Charge Transfer as a Substrate for Conductive Materials

In order to obtain a flexible aerogel substrate for conductive materials used in the electrode, polydopamine-anchored cellulose nanofiber (PDA@CNF) was introduced into a polyethylene imine-poly(vinyl alcohol) (PEI-PVA) cross-linking network which used 4-formylphenylboronic acid (4FPBA) as bridge. The incorporation of rigid CNF as a structural scaffold effectively improved the pore architecture of the aerogel, potentially providing substantial advantages for the infiltration and deposition of conductive materials. Additionally, the outstanding stability and flexibility exhibited by the aerogel in aqueous solutions suggest its significant potential for applications in flexible electrodes. Furthermore, electrochemical experiments showed that the rapid pathway formed between PDA and PEI could enhance the charge-transfer rate within the aerogel substrate. It is anticipated that such an enhancement would significantly benefit the electrochemical attributes of the electrode. Inspired by mussels, our introduced PDA-anchored rigid CNF into flexible polymer networks to fabricate aerogel substrates for electrode materials. This study would contribute to the development and utilization of flexible electrodes while reducing carbon footprint in energy production and conversion processes.

Transparent, super stretchable, freezing-tolerant, self-healing ionic conductive cellulose based eutectogel for multi-functional sensors

In this study, we propose a non - toxic and low-cost fabrication of cellulose-based eutectogel through the ZnCl2/H2O/H3PO4 deep eutectic solvent (DES) to dissolve cellulose followed by free-radical polymerization of acrylamide. Particularly, the introduction of cellulose enhances the mechanical properties of eutectogels while eliminating the environmental concerns of the traditional nanocellulose fabrication process. Owing to the dynamic transfer of ions in the eutectogel network, the prepared eutectogels exhibit adjustable conductivity (0.9- 1.37 Sm-1, 15 °C) and stretching sensitivity (Gauge factor = 5.4). The resulting DES - cellulose-based eutectogels (DCEs) exhibited ultra stretchability (4086 %), high toughness (261.3 MJ/m3), excellent ionic conductivity (1.64 Sm-1, 20 °C), high transparency (>85 %), outstanding antifreezing performance (<-80 °C), and other comprehensive characteristics. The DCEs had been proven to have multiple sensitivities to external stimuli, like temperature, strain, and pressure. As a result, the DCEs can be assembled into multifunctional sensors. Moreover, this work also demonstrated the satisfactory performance of DCEs in flexible electroluminescent devices. The low cost and high efficiency made the preparation method of this experiment an efficient strategy for developing high-performance cellulose-based eutectogels, which would greatly promote the application of such materials in areas such as artificial skin for soft robots and other wearable devices.

A High-Stretching, Rapid-Self-Healing, and Printable Composite Hydrogel Based on Poly(Vinyl Alcohol), Nanocellulose, and Sodium Alginate

Hydrogels with excellent flexibility, conductivity, and controllable mechanical properties are the current research hotspots in the field of biomaterial sensors. However, it is difficult for hydrogel sensors to regain their original function after being damaged, which limits their practical applications. Herein, a composite hydrogel (named SPBC) of poly(vinyl alcohol) (PVA)/sodium alginate (SA)/cellulose nanofibers (CNFs)/sodium borate tetrahydrate was synthesized, which has good self-healing, electrical conductivity, and excellent mechanical properties. The SPBC0.3 hydrogel demonstrates rapid self-healing (<30 s) and achieves mechanical properties of 33.92 kPa. Additionally, it exhibits high tensile strain performance (4000%). The abundant internal ions and functional groups of SPBC hydrogels provide support for the good electrical conductivity (0.62 S/cm) and electrical response properties. In addition, the SPBC hydrogel can be attached to surfaces such as fingers and wrists to monitor human movements in real time, and its good rheological property supports three-dimensional (3D) printing molding methods. In summary, this study successfully prepared a self-healing, conductive, printable, and mechanically superior SPBC hydrogel. Its suitability for 3D-printing personalized fabrication and outstanding sensor properties makes it a useful reference for hydrogels in wearable devices and human motion monitoring.

Using bacterial cellulose to bridge covalent and physical crosslinks in hydrogels for fabricating multimodal sensors

Network optimization is vital for the polysaccharide based hydrogels with multiple crosslinks. In this study, we developed a 'two-step' strategy to activate synergistic effect of chemical and physical crosslinks using a poly (vinyl alcohol) (PVA)/bacterial cellulose (BC) hydrogel as a template. The BC nanofibers, on the one hand, acted as nucleating agents, participating in the crystallization of PVA, and on the other hand, were also involved in the formation of boronic ester bond, anchored with the PVA chains via chemical bonding. Therefore, the existence of BC nanofibers, as 'bridge', linked the crystalline regions and amorphous parts of PVA together, associating the two characteristic crosslinks, which was conducive to load transfer. The mechanical properties of resultant hydrogels, including the tensile elongation and strength, as well as fracture toughness, were significantly improved. Moreover, the dually cross-linked hydrogels possessed ionic conductivity, which was sensitive to the tensile deformation and environmental temperature. This study clarifies a unique role of BC nanofibers in hydrogels, and proposes an effective approach to construct multiple networks in the nanocellulose reinforced PVA hydrogels.

A co-type ductile film with high tensile strength and fast self-healing properties for shaped fruit preservation

Traditional petroleum-based plastics have high energy consumption, require professional equipment, are non-degradable after use, and lack antibacterial properties, making it impossible to achieve long-lasting freshness in fruits and vegetables. Herein, we report a novel co-type film-forming method with low energy consumption and without production equipment, which uses PVA-borax gel as a substrate and adds a certain proportion of CMC and TA to prepare multifunctional CMC/TA@PVA-borax composite hydrogels (CTPB). The dynamic borax ester bonding and hydrogen bonding in the CTPB hydrogel results in an ultra-high tensile strength of more than 5500% and rapid self-healing within 8 s. Interestingly, hydrogels can be arbitrarily shaped and stretched like play dough and thus can be stretched into ductile films by co-type film formation. The antimicrobial properties of the hydrogel film can be attributed to the synergistic effects of TA and borax. The mussel structure of TA allows the hydrogel film to adhere directly to different surfaces for more effective bacterial killing. In addition, the hydrogel film has a high level of biosafety and biodegradability and shows good performance in fruit storage. This study provides a convenient and low-energy method for the preparation of films, which in part reduces the increasing environmental pollution caused by petroleum-based plastics.

Cellulose nanocrystals boosted hydrophobically associated self-healable conductive hydrogels for the application of strain sensors and electronic devices

Currently, hydrogel-based flexible devices become hot areas for scientists in the field of electronic devices, artificial intelligence, human motion detection, and electronic skin. These devices show responses to external stimuli (mechanical signals) and convert them into electrical signals (resistance, current, and voltage). However, the applications of the hydrogel-based sensor are hampered due to low mechanical properties, high time response, low fatigue resistance, low self-healing nature, and low sensing range. Herein, a strain sensing conductive hydrogel constructed from the CNCs (cellulose nanocrystal) reinforced, in which acrylamide and butyl acrylate work as hydrophilic and hydrophobic monomers respectively. The incorporation of CNCs in the polymeric system has a direct effect on their mechanical properties. The hydrogel having a high amount of CNCs (C4), its fracture stress and fracture strain reached 371.2 kPa and 2108 % respectively as well as self-healing of C4 hydrogel Broke at 499 % strain and bore 197 kPa stress. The elastic behavior of the hydrogels was confirmed by the rheological parameter frequency sweep and strain amplitude. Besides this our designed hydrogel shows an excellent response to deformation with conductivity 420 mS m-1, shows response to small strain (10 %) and large (400 %) strain, and has excellent anti-fatigue resistance with continuous stretching for 700 s at 300 % strain, with 140 msec response time, and gauge factor 7.4 at 750 % strain. The C4 hydrogel can also work as electronic skin when it is applied to different joints like the finger, elbow, neck, etc. The prepared hydrogel can also work as an electronic pen when it is worn to a plastic pen cover.

Co-immobilizing laccase-mediator system by in-situ synthesis of MOF in PVA hydrogels for enhanced laccase stability and dye decolorization efficiency

The laccase mediator system (LMS) with a broad substrate range has attracted much attention as an efficient approach for water remediation. However, the practical application of LMS is limited due to their high solubility, poor stability and low reusability. Herein, the bimetallic Cu/ZIFs encapsulated laccase was in-situ grown in poly(vinyl alcohol) (PVA) polymer matrix. The PVA-Lac@Cu/ZIFs hydrogel was formed via one freeze-thawing cycle, and its catalytic stability was significantly improved. The mediator was further co-immobilized on the hydrogel, and this hierarchically co-immobilized ABTS/PVA-Lac@Cu/ZIFs hydrogel could avoid the continuous oxidation reaction between laccase and redox mediators. The co-immobilized LMS biocatalyst was used to degrade malachite green (MG), and the degradation rate was up to 100 % within 4 h. More importantly, the LMS could be recycled synchronously from the dye solutions and reused to degrade MG multiple times. The degradation rate remained above 69.4 % after five cycles. Furthermore, the intermediate products were detected via liquid chromatography-mass spectrometry, and the potential degradation pathways were proposed. This study demonstrated the significant potential of utilizing the MOF nanocrystals and hydrogel as a carrier for co-immobilized LMS, and the effective reuse of both laccase and mediator was promising for laccase application in wastewater treatment.

Cellulose nanofiber hydrogel with high conductivity electrolytes for high voltage flexible supercapacitors

Although flexible double layer capacitors based on hydrogels overcome the drawbacks of commercial double layer capacitors such as low safety and non-deformability, it is still considered as attractive challenges to achieve high conductivity for hydrogel electrolytes as well as high operating voltages for hydrogel flexible supercapacitors. In this paper, ion migration channels were engineered by immobilizing positive and negative charges on polymer skeleton and dispersing cellulose nanofibers in the polymerized polyelectrolyte network, providing ultra-high ionic conductivity (103 mS cm-1). In addition, K3[Fe(CN)6] was introduced through a soaking method, leading to redox reactions on the surface of carbon electrode during charging and discharging, supporting a relatively wide voltage window (1.8 V). Moreover, the specific capacitance at high current remained 55 % of the specific capacitance at low current, indicating excellent rate performance. In addition, the device displayed high cycling stability (80.05 % after 10,000 cycles). Notably, we successfully light up the red LED with only one device. Accordingly, this work provides a feasible design concept for the development of cellulose nanofibers (CNF) hydrogel-based solid-state electrolyte with high conductivity for flexible supercapacitors with wide potential window and high energy density.

Highly stretchable polyvinyl alcohol composite conductive hydrogel sensors reinforced by cellulose nanofibrils and liquid metal for information transmission

Conductive hydrogels have received widespread attention in the field of flexible sensors. However, a single network structure inside the hydrogel sensor usually makes it difficult to bear larger mechanical loadings, greatly limiting practical applications. Developing a recoverable conductive hydrogel sensor with high toughness and adaptability is still challenging. Herein, a high-performance polyvinyl alcohol (PVA)-based conductive composite hydrogel was constructed, assisted by green cellulose nanofibrils (CNFs), magnesium chloride (MgCl2), ethylene glycol (EG), and liquid metal (LM). The synergistic effects between CNFs and LM enhanced the network structure inside the recoverable hydrogel. This resulted in an excellent tensile strength of 3.86 MPa with an elongation at break of as high as 918.4 % and compressive strength of 4.04 MPa at 80 % strain. In addition, the conductive network composed of MgCl2 and LM endowed the hydrogel good electrical conductivity. Moreover, it could be used as a flexible strain sensor for various application scenarios, e.g., micro-stress monitoring (water droplet falling) and information encryption transmission of Morse code. Such uniqueness will provide a design strategy for developing a new generation of hydrogel sensors.

Hydrogels for Chemical Sensing and Biosensing

The development and synthesis of hydrogels for chemical and biosensing are of great value. Hydrogels can be tailored to its own physical structure, chemical properties, biocompatibility, and sensitivity to external stimuli when being used in a specific environment. Herein, hydrogels and their applications in chemical and biosensing are mainly covered. In particular, it is focused on the manner in which hydrogels serve as sensing materials to a specific analyte. Different types of responsive hydrogels are hence introduced and summarized. Researchers can modify different chemical groups on the skeleton of the hydrogels, which make them as good chemical and biosensing materials. Hydrogels have great application potential for chemical and biosensing in the biomedical field and some emerging fields, such as wearable devices.

Multimodal and flexible hydrogel-based sensors for respiratory monitoring and posture recognition

The accurate monitoring of respiratory events and human motion states holds paramount importance in the realm of health surveillance and disease prognostication. An exquisitely precise, multifaceted, portable, and environmentally resilient sensor designed for health monitoring would undeniably be of utmost desirability, despite its persisting as a formidable challenge. Here, we propose a breath monitoring and posture recognition system that utilizes hydrogel electrolytes based on polyvinyl alcohol, sodium alginate, and starch, suitable for supercapacitors and multimodal wearable sensors. The multimodal smart sensors can independently detect mechanical and thermal changes through the output signals of capacitance and resistance, respectively. Moreover, we have cultivated an artificial neural network to achieve a finger-pressing posture recognition accuracy of up to 99.259%. Our hydrogel sensors have also been successfully employed in the diagnosis of obstructive sleep apnea syndrome. The flexible electronic device derived from this study exhibit a plethora of functionalities, thereby affording a novel perspective for the design and fabrication of advanced flexible electronic contrivances that find applications across diverse domains such as medicine and virtual reality.

Establishing a Corrugated Carbon Network with a Crack Structure in a Hydrogel for Improving Sensing Performance

Electronic conductive hydrogels have prompted immense research interest as flexible sensing materials. However, establishing a continuous electronic conductive network within a hydrogel is still highly challenging. Herein, we develop a new strategy to establish a continuous corrugated carbon network within a hydrogel by embedding carbonized crepe paper into the hydrogel with its corrugations perpendicular to the stretching direction using a casting technique. The corrugated carbon network within the as-prepared composite hydrogel serves as a rigid conductive network to simultaneously improve the tensile strength and conductivity of the composite hydrogel. The composite hydrogel also generates a crack structure when it is stretched, enabling the composite hydrogel to show ultrahigh sensitivity (gauge factor = 59.7 and 114 at strain ranges of 0-60 and 60-100%, respectively). The composite hydrogel also shows an ultralow detection limit of 0.1%, an ultrafast response/recovery time of 75/95 ms, and good stability and durability (5000 cycles at 10% strain) when used as a resistive strain sensing material. Moreover, the good stretchability, adhesiveness, and self-healing ability of the hydrogel were also effectively retained after the corrugated carbon network was introduced into the hydrogel. Because of its outstanding sensing performance, the composite hydrogel has potential applications in sensing various human activities, including accurately recording subtle variations in wrist pulse waves and small-/large-scale complex human activities. Our work provides a new approach to develop economical, environmentally friendly, and reliable electronic conductive hydrogels with ultrahigh sensing performance for the future development of electronic skin and wearable devices.

Self-strengthening and conductive cellulose composite hydrogel for high sensitivity strain sensor and flexible triboelectric nanogenerator

Triboelectric nanogenerators (TENGs) as promising energy harvesting devices have gained increasing attention. However, the fabrication of TENG simultaneously meets the requirements of green start feedstock, flexible, stretchable, and environmentally friendly remains challenging. Herein, the hydroxyethyl cellulose macromonomer (HECM) simultaneously bearing acrylate and hydroxyl groups was first synthesized and used as a crosslinker to prepare the chemically and physically dual-crosslinked cellulose composite hydrogel for an electrode material of stretchable TENG. Meanwhile, the in-situ polymerization of pyrrole endowed the hydrogel with satisfactory conductivity of 0.40 S/m. More impressively, the synergies of the cellulose rigid skeleton and the construction of the dual-crosslinking network significantly improved the mechanical toughness, and the hydrogel exhibited excellent self-strengthening through cyclic compression mechanical training, the self-strengthening efficiency reached 124.7 % after 10 compression cycles. Given these features, the hydrogel was used as wearable strain sensors with extremely high sensitivity (GF = 3.95) for real-time monitoring human motions. Additionally, the hydrogel showed practical applications in stretchable H-TENG for converting mechanical energy into electric energy to light LEDs and power a digital watch, and in self-powered wearable sensors to distinguish human motions and English letters. This work provided a promising strategy for fabricating sustainable, eco-friendly energy harvesting and self-powered electronic devices.

A Transparent Poly(vinyl alcohol) Ion-Conducting Organohydrogel for Skin-Based Strain-Sensing Applications

The increasing demand for cost-efficient and user-friendly wearable electronic devices has led to the development of stretchable electronics that are both cost-effective and capable of maintaining sustained adhesion and electrical performance under duress. This study reports on a novel physically crosslinked poly(vinyl alcohol) (PVA)-based hydrogel that serves as a transparent, strain-sensing skin adhesive for motion monitoring. By incorporating Zn2+ into the ice-templated PVA gel, a densified amorphous structure is observed through optical and scanning electron microscopy, and it is found that the material can stretch up to 800% strain according to tensile tests. Fabrication in a binary glycerol:water solvent results in electrical resistance in the kΩ range, a gauge factor of 0.84, and ionic conductivity on the scale of 10-4 S cm-1 , making it a potentially low-cost candidate for a stretchable electronic material. This study characterizes the relationship between improved electrical performance and polymer-polymer interactions through spectroscopic techniques, which play a role in the transport of ionic species through the material.

Ultrastrong and Tough Urushiol-Based Ionic Conductive Double Network Hydrogels as Flexible Strain Sensors

Ionic conductive hydrogels have attracted increasing research interest in flexible electronics. However, the limited resilience and poor fatigue resistance of current ionic hydrogels significantly restrict their practical application. Herein, an urushiol-based ionic conductive double network hydrogel (PU/PVA-Li) was developed by one-pot thermal initiation polymerization assisted with freeze-thaw cycling and subsequent LiCl soaking. Such a PU/PVA-Li hydrogel comprises a primary network of covalently crosslinked polyurushiol (PU) and a secondary network formed by physically crosslinked poly(vinyl alcohol) (PVA) through crystalline regions. The obtained PU/PVA-Li hydrogel demonstrates exceptional mechanical properties, including ultrahigh strength (up to 3.4 MPa), remarkable toughness (up to 1868.6 kJ/m3), and outstanding fatigue resistance, which can be attributed to the synergistic effect of the interpenetrating network structure and dynamic physical interactions between PU and PVA chains. Moreover, the incorporation of LiCl into the hydrogels induces polymer chain contraction via ionic coordination, further enhancing their mechanical strength and resilience, which also impart exceptional ionic conductivity (2.62 mS/m) to the hydrogels. Based on these excellent characteristics of PU/PVA-Li hydrogel, a high-performance flexible strain sensor is developed, which exhibits high sensitivity, excellent stability, and reliability. This PU/PVA-Li hydrogel sensor can be effectively utilized as a wearable electronic device for monitoring various human joint movements. This PU/PVA-Li hydrogel sensor could also demonstrate its great potential in information encryption and decryption through Morse code. This work provides a facile strategy for designing versatile, ultrastrong, and tough ionic conductive hydrogels using sustainable natural extracts and biocompatible polymers. The developed hydrogels hold great potential as promising candidate materials for future flexible intelligent electronics.

Facile fabrication of strong and conductive cellulose hydrogels with wide temperature tolerance for flexible sensors

Cellulose-based ionic conductive hydrogels (ICHs) have found extensive applications in flexible electronics and multifunctional sensors. However, simultaneous realization of sufficient conductivity, superior mechanical property and extreme environment tolerance for ICHs remains to be a huge challenge. In this work, a facile one-pot approach was developed to fabricate ICHs by directly dissolving cotton linter cellulose and polyvinyl alcohol (PVA) in a concentrated ZnCl₂ solution. By regulating the content of PVA in ICHs, the optimal hydrogel (Gel-5) exhibits a tensile strength of 0.30 MPa, a compressive strength of 2.05 MPa and a conductivity of 8.16 S m^-1. Moreover, the resulting dual-network ICHs present high transparency, good thermal reversibility and desirable ionic conductivity. Due to the high concentration of inorganic salts in the porous dual-network structure, the ICH presents good anti-drying and anti-freezing (as low as -90 °C) properties. Such hydrogel can be assembled into multi-functional sensors for human motion and temperature monitoring, and they demonstrate durable sensitivity, cycling stability in a wide operating temperature. This work will shed light on the design of cellulose-based hydrogels with good ionic conductivity and mechanical performance under extreme conditions.

Reinforcement of Nanocomposite Hydrogel with Dialdehyde Cellulose Nanofibrils via Physical and Double Network Crosslinking Synergies

Preparation of tough and high-strength hydrogels for water plugging in oil fields with an easy-scalable method is still considered to be a challenge. In this study, dialdehyde cellulose nanofibril (DA-CNF) prepared by sodium periodate oxidation, polyamine, 2-acrylamido-2-methylpropane sulfonic acid (AMPS) with sulfonate groups and Acrylamide (AM) as raw materials, CNF reinforced nanocomposite hydrogels were prepared in one step by in-situ polymerization. The tensile strength, and texture stability of the obtained nanocomposite hydrogel were determined. The results showed that the tensile strength and toughness of the obtained nanocomposite hydrogel increased four times compared with control sample due to physical and chemical double crosslinking synergies. Moreover, the texture intensity of DA-CNFs reinforced hydrogel still maintains high stability and strength performance under high salinity conditions. Therefore, DA-CNF reinforced hydrogel has potential application value in both normal and high-salinity environments in oil recovery.

Dual-network polyvinyl alcohol/polyacrylamide/xanthan gum ionic conductive hydrogels for flexible electronic devices

Ionic conductive hydrogels (ICHs) have received widespread attention as an ideal candidate for flexible electronic devices. However, conventional ICHs failed in widespread applications due to their inability to simultaneously possess high toughness, high ionic conductivity, and anti-freezing properties. Here, polyvinyl alcohol (PVA) and polyacrylamide (PAAm) were first dissolved in the zinc chloride solution, in which zinc ions (Zn²⁺) act as ionic cross-linkers and conducting ions, followed by the introduction of xanthan gum (XG) with a unique structure of trisaccharide side chains into the PVA/PAAm semi-interpenetrating network to prepare a dual-network ICHs (refers as PPXZ). Enabled by the synergistic effect of intermolecular chemical covalent cross-linking and physical cross-linking, PPXZ hydrogels exhibit significantly improved mechanical properties without sacrificing electrical conductivity. Furthermore, PPXZ hydrogels are successfully applied to flexible electronic devices, such as strain sensors and zinc ion hybrid supercapacitors, exhibiting satisfactory sensing sensitivity and cycling stability at a wide temperature range, respectively. Even at a high current density (10 A g^-1), the capacity of the supercapacitor retains 88.24 % after 10,000 cycles. This strategy provides new insight for ICHs in wide temperature-applied flexible electronic devices.

Multi-physics coupling reinforced polyvinyl alcohol/cellulose nanofibrils based multifunctional hydrogel sensor for human motion monitoring

Ionic conductive hydrogels have been widely used for sensor, energy storage and human-machine interface. To address the problems of the traditional ionic conductive hydrogels fabricated with the soaking method, such as the lack of frost resistance, poor mechanical properties, time-consuming and chemical-wasting, herein, a multi-physics crosslinking reinforced strong, anti-freezing and ionic conductive hydrogel sensor is fabricated utilizing the tannin acid-Fe₂(SO₄)₃ through the simple one-pot freezing-thawing process at low electrolyte concentration. The results show that the P₁₀C_0.4T₈-Fe₂(SO₄)₃ (PVA_10%CNF_0.4%TA_8%-Fe₂(SO₄)₃) displayed better mechanical property and ionic conductivity due to hydrogen bonding and coordination interaction. The tensile stress reaches up to 0.980 MPa (570 % strain). Moreover, the hydrogel presents excellent ionic conductivity (0.220 S⋅m^-1 at room temperature), anti-freezing performance (0.183 S⋅m^-1 at -18 °C), large gauge factor (1.75), excellent sensing stability, repeatability, durability and reliability. This work paves a way for preparing mechanical strong and anti-freezing hydrogel based on multi-physics crosslinking with one-pot freezing-thawing process.

Anti-freezing, recoverable and transparent conductive hydrogels co-reinforced by ethylene glycol as flexible sensors for human motion monitoring

Wearable flexible sensors based on conductive hydrogels have received extensive attention in the fields of electronic skin and smart monitoring. However, conductive hydrogels contain a large amount of water, which greatly affects their performances in harsh environments. It is therefore necessary to prepare hydrogel sensors that are stable at low temperatures. Herein, metal ions (MgCl₂) and ethylene glycol (EG) were combined with polyvinyl alcohol (PVA) to obtain a conductive PVA/EG hydrogel with tensile strength and elongation at break of 1.1 MPa and 442.3 %, respectively, which could withstand >6000-fold its own weight. The binary solvent system composed of water and EG contributed to the excellent anti-freezing properties and long-term storage (>1 week), flexibility, and stability of the hydrogel even at -20 °C. The wearable PVA/EG hydrogel as a flexible sensor possessed desirable sensing performances with a competitive GF value of 0.725 and fatigue resistance (50 cycles) when used to monitor various human motions and physiological signals. Overall, this hydrogel sensor shows strong potential for application in the fields of human motion monitoring, written information sensing, and information encryption and transmission.

Highly stretchable, adhesive, and biocompatible hydrogel platforms of tannic acid functionalized spherical nanocellulose for strain sensors

The development of multifunctional wearable electronic devices has received considerable attention because of their attractive applications. However, integrating multifunctional abilities into one component remains a challenge. To address this, we have developed a tannic acid-functionalized spherical nanocellulose/polyvinyl alcohol composite hydrogel using borax as a crosslinking agent for strain-sensing applications. The hydrogel demonstrates improved mechanical and recovery strengths and maintains its mechanical strength under freezing conditions. The hydrogels show ultra-stretching, adhesive, self-healing, and conductive properties, making them ideal candidates for developing strain-based wearable devices. The hydrogel exhibits good sensitivity with a 4.75 gauge factor. The cytotoxicity of the developed hydrogels was monitored with human dermal fibroblast cells by WST-8 assay in vitro. The antibacterial potential of the hydrogels was evaluated using Escherichia coli. The hydrogels demonstrate enhanced antibacterial ability than the control. Therefore, the developed multifunctional hydrogels with desirable properties are promising platforms for strain sensor devices.

Multiply cross-linked poly(vinyl alcohol)/cellulose nanofiber composite ionic conductive hydrogels for strain sensors

Building multiple chemical crosslinks is an effective strategy to improve mechanical properties and to diversify final application of polysaccharide nanoparticles reinforced poly(vinyl alcohol) (PVA) physical hydrogels. In this work, PVA/cellulose nanofibers (CNFs) were used as composite substrate to fabricate ionic conductive hydrogels for strain sensor. Three types of characteristic crosslinks, including chemical crosslinking via boronic ester covalent bonds only, and with additional metal coordination bonding, as well as coexistence of physical crosslinks via PVA crystallites and aforementioned two kinds of chemical crosslinks, were constructed. The sample with triple crosslinks has superior mechanical strength and resistance to fatigue, and the polydopamine/Fe³⁺ ratio act as key to tune final performance because double-network structure prefers to form as Fe³⁺ is superfluous, while dual-crosslink one forms in the case of insufficient Fe³⁺. As-optimized ionic conductive hydrogel is suitable as strain sensor for probing human motions. This work provides an interesting insight into the network structure and property regulation for PVA/CNF composite hydrogels with multiple crosslinks.

Flexible Stretchable, Dry-Resistant MXene Nanocomposite Conductive Hydrogel for Human Motion Monitoring

Conductive hydrogels with high electrical conductivity, ductility, and anti-dryness have promising applications in flexible wearable electronics. However, its potential applications in such a developing field are severely hampered by its extremely poor adaptability to cold or hot environmental conditions. In this research, an "organic solvent/water" composite conductive hydrogel is developed by introducing a binary organic solvent of EG/H₂O into the system using a simple one-pot free radical polymerization method to create Ti₃C₂T_X MXene nanosheet-reinforced polyvinyl alcohol/polyacrylamide covalently networked nanocomposite hydrogels (PAEM) with excellent flexibility and mechanical properties. The optimized PAEM contains 0.3 wt% MXene has excellent mechanical performance (tensile elongation of ~1033%) and an improved modulus of elasticity (0.14 MPa), a stable temperature tolerance from -50 to 40 °C, and a high gauge factor of 10.95 with a long storage period and response time of 110 ms. Additionally, it is worth noting that the elongation at break at -40 °C was maintained at around 50% of room temperature. This research will contribute to the development of flexible sensors for human-computer interaction, electronic skin, and human health monitoring.

Fabrication of Densified Rice Husk by Sequential Hot-Compressed Water Treatment, Blending with Poly(vinyl alcohol), and Hot Pressing

Processing agricultural wastes into densified materials to partially substitute wooden product production is significant for reducing the consumption of forest resources. This work proposes the fabrication of high-strength rice husk (RH)-based composite materials with poly(vinyl alcohol) (PVA) via densification by hot pressing. RH was pretreated in hot-compressed water (HCW) prior to pulverization and blending with PVA or PVA/glycerol (GL). The incorporation of PVA greatly improved the strength, toughness, and waterproofness of the composite plate, which was discussed with the help of a variety of composite characterizations. The tensile strength, flexural strength, and toughness of a composite of HCW-treated RH, PVA, and GL with a mass ratio of 80:20:2 were 42, 81 MPa, and 5.9 MJ/m³, respectively. The HCW treatment and blending with PVA and GL improved those properties of the hot-pressed original RH plate by factors of 2.5, 2.3, and 6.7, respectively, and reduced the water uptake and swelling ratio in water by 57 and 53%, respectively, despite the hydrophilic nature of PVA and GL. Altogether, this work outlines a valuable and sustainable approach to the efficient utilization of agricultural wastes.

Biomimetic Microstructured Antifatigue Fracture Hydrogel Sensor for Human Motion Detection with Enhanced Sensing Sensitivity

Antifatigue fracture performance and high sensing sensitivity are key characteristics for hydrogel sensors used in flexible electronic applications. Herein, inspired by human muscle tissues and epidermal skin tissues, an effective and straightforward strategy is proposed to fabricate hydrogel sensors for detecting human motion with antifatigue fracture performance and high sensing sensitivity. The crystalline regions and orientation along the stretching direction of cellulose nanofiber@carbon nanotube nanohybrids in the hydrogels provide antifatigue fracture performance (the crack does not expand after 2000 stretching cycles, and the fatigue threshold was calculated to be 187 J/m2), which protects hydrogels from severe damage during long-term use. In addition, the microstructured surfaces of the hydrogels with a random height distribution increase the contact area and improve the response to weak stimuli, resulting in a sensing sensitivity of 1.11 kPa-1, 18 times higher than that of a flat hydrogel. This sensing sensitivity is higher than those of most of the hydrogel-based pressure sensors that have been reported earlier. By integrating antifatigue fracture performance and enhanced sensing sensitivity, biomimetic microstructured hydrogel sensors show great potential for use in future flexible electronic applications.

Perspective about Cellulose-Based Pressure and Strain Sensors for Human Motion Detection

High-performance wearable sensors, especially resistive pressure and strain sensors, have shown to be promising approaches for the next generation of health monitoring. Besides being skin-friendly and biocompatible, the required features for such types of sensors are lightweight, flexible, and stretchable. Cellulose-based materials in their different forms, such as air-porous materials and hydrogels, can have advantageous properties to these sensors. For example, cellulosic sensors can present superior mechanical properties which lead to improved sensor performance. Here, recent advances in cellulose-based pressure and strain sensors for human motion detection are reviewed. The methodologies and materials for obtaining such devices and the highlights of pressure and strain sensor features are also described. Finally, the feasibility and the prospects of the field are discussed.

Optically active plasmonic cellulose fibers based on Au nanorods for SERS applications

Cellulose might be a promising material for surface-enhanced Raman scattering (SERS) substrates due to its wide availability, low cost, ease of fabrication, high flexibility and low optical activity. This work shows, for the first time development of the cellulose-based substrate, that owes its SERS activity to the presence of gold nanorods in its internal structure, and not only on the surface, as it is shown elsewhere, thus ensuring superior stability of the obtained material. This flexible cellulose-based substrate exhibiting plasmonic activity, provide easy and reproducible detection of different analytes via SERS technique. The substrate was prepared by introduction of gold nanorods into the cellulose fibers matrix using an eco-friendly process based on N-Methylmorpholine-N-Oxide. Au-modified cellulose fibers were used for the detection of p-Mercaptobenzoic acid and Bovine Serum Albumin by the SERS method. The obtained results show that this substrate offers large signal enhancement of 6-orders of magnitude, and high signal reproducibility with a relative standard deviation of 8.3%. Additionally, washing tests (90 °C, 20 h) showed superior stability of the as prepared plasmonic fibers, thus proving the good reusability of the substrates and the long shelf life.

Reduced graphene oxide/cellulose nanocrystal composite films with high specific capacitance and tensile strength

Due to the environmental degradation and energy depletion, the strategy for fabricating high-performance supercapacitor electrode materials based on graphene and nanocellulose has received great attention. Herein, an environmentally friendly reduced graphene oxide (RGO)/cellulose nanocrystal (CNC) composite conductive film was prepared using L-ascorbic acid (L-AA) as the reductant of graphene oxide (GO). Based on chemical structure analysis, L-AA was proved to be an effective reductant to remove oxygen containing groups of GO. Through microstructure observation, a unique stacking structure of CNC and RGO was observed, which could be largely attributed to the hydrogen bond interaction. Furthermore, the effect of CNC amount on the performance of RGO/CNC composite films was also systematically investigated. Particularly, the addition of CNC was found to exert a positive effect on the tensile strength, which might be mainly due to a mass of hydrogen bonds between the CNCs. Meanwhile, the RGO/CNC composite conductive film featured ideal electrical double-layer capacitive (EDLC) behavior, exhibiting a gravity specific capacitance of 222.5 F/g and tensile strength of 32.17 MPa at 20 wt% CNC content. Therefore, the RGO/CNC composite conductive films may hold great promise for environmentally friendly electrode materials of supercapacitors and flexible electrical devices.

Cellulose nanocrystal reinforced conductive hydrogels with anti-freezing properties for strain sensors

High strength hydrogels with frost resistance can be used as human body sensors in low temperature environment.